I actually played some GP last night on my Japanese copy of SMK. All my experience with time trialling couldn't quite prepare me for Rainbow Road at 150cc. I ran off the road so many times, I finished out of the top four on my first
four attempts before finishing first on my very last try. (What did I do different that time? Simple, when I had the lead on the final lap I stopped trying to hop around turns and drove like a complete n00b would drive in this game, slowing the Kart down on every turn)
GP is more different from TT than PAL time trialling is from NTSC time trialling. The startboost timing is different. Your Kart moves
much faster on 150cc especially after picking up a few coins. (Heck, even on 100cc, I think your Kart is faster than on TT) Your turns are much wider, and this is what makes RR with Bowser on 150cc such a pain in the ass. There are also extra jump plates on RR in GP that aren't there in TT, but this has comparitively little effect on the track's increased difficulty.
And, of course, there's the items, and learning how to use them skillfully to take the best shortcuts on each course, or to hamper your opponents. Obviously the item selection is going to be luck-oriented; you can't realistically expect to get five feathers on BC2 each time you race on it. Also, even on 150cc you might end up lapping the slower computer racers, who always got in my way especially on turns (I actually played on all three speed settings, to get every gold trophy in the game; you
will lap the computer on 100c and
especially 50cc because they are
very slow)
I can imagine that competing at the top level in 150cc GP at the CDM requires a
lot of training because there is so much that you have to get used to that's different from the Time Trial. I'm sure if I practiced enough, I too could learn to control Bowser at 150cc top speed and make it around RR without falling off. With the higher speed of 150cc GP, it would look absolutely awesome, and I'd be able to blow away players who are only accustomed to Bowser/DK Jr.'s TT handling.
And it would truly take a special kind of Karter to excel at both GP
and TT (not to mention Battle!), especially at the same CDM. A high-level Battle game would probably be awesome to watch as well, especially if one player is Bowser/DK Jr. and skillfully navigates the maze-like courses at top speed, chasing down his opponent like the red ghost in a game of
Pac-Man.

I did have a few good runs last night, though. I drove times on BC1 and GV2 that were much faster than my TT PRs (possibly even the WRs), although those two tracks have zippers added in GP. However, one other track gave me a lot of trouble, and that was BC3, another track where I had multiple rank-outs before finally finishing in the top four and moving on to MC4. This and RR are far more unforgiving for Bowser players than the rest of the tracks, between the Thwomps and the risk of running off the track and having to be reeled in by Lakitu.
I think DK Jr. might be a better choice than Bowser for 150cc GP, because Bowser always has to deal with the two Mario Bros. close by, both of which can turn invincible and spin you out when you approach them. DK only has to worry about Toad and his poison mushrooms.
All you Time Triallers out there, if you're looking for a new challenge, try setting time records in 150cc GP. I promise, your SMK driving skills will be tested to the absolute max.